Pressure vessels for various purposes such as abrasive blasting typically have a fill hatch which is closed and locked when the vessel is under pressure, but which can be opened to add material to the pressure vessel and/or for interior inspection and work. While such hatches typically have locking mechanisms, there can be safety concerns if the locks are prematurely disengaged while the vessel is still pressurized. The present apparatus provides an automatic safety lock responsive to internal pressure in a pressure vessel or pressurized tank to minimize the ability to prematurely open the hatch.